The selective estrogen receptor modulator (SERM) tamoxifen (TAM) is the most widely used endocrine therapy for the treatment and prevention of estrogen receptor alpha (ERalpha) positive breast cancer. With adjuvant therapy, however, initially TAM-sensitive tumors become TAM-resistant. The mechanism behind such acquired TAM resistance is unknown and biomarkers of TAM-response may be useful to monitor clinical response. MicroRNAs (miRNAs) are a class of naturally-occurring, small, non-coding RNA molecules that are related to, but distinct from, small interfering RNAs (siRNAs). miRNAs are involved in regulating the translation and processing of mRNAs, usually by binding to the 3'untranslated region of target mRNAs and targeting the mRNA transcript to be degraded. The human genome is thought to contain >400 miRNAs. Abberant patterns of miRNA expression have been recently implicated in human breast cancer and one study showed that miRNAs were differentially expressed in concordance with other well-established markers of breast cancer stage and patient prognosis including ERalpha and progesterone receptor expression, tumor stage, number of positive lymph nodes, and vascular invasion. However, to date, no one has examined whether estradiol (E2) or selective ER modulators/antiestrogens affect the pattern of miRNA expression in human breast cancer or whether miRNA expression patterns correlate with acquired antiestrogen-resistance. Specific Aim 1 is to identify miRNAs that are regulated by estradiol (E2) and 4-hydroxytamoxifen (4-OHT) in antiestrogen-sensitive MCF-7 breast cancer cells. Aim 1 tests the hypothesis that E2 and 4-OHT differentially regulate the expression of miRNAs in human breast cancer cells. Further, we suggest these miRNAs in turn regulate the mRNA stability of genes encoding proteins involved in differentiation and cell proliferation. Specific Aim 2 tests the hypothesis that miRNA expression is dysregulated in antiestrogen/TAM-resistant versus -sensitive breast cancer cells. Specific Aim 3 is to determine if the E2-regulated and 4-OHT-regulated miRNAs identified in breast cancer cell lines show aberrant expression in human breast tumors and correlate with patient response to tamoxifen therapy. It is our long term hope that miRNAs may provide novel biomarkers and new insights into the mechanisms by which breast tumors gain TAM/antiestrogen-resistance and become invasive and metastatic.